Center channel speakers

What range should a good center channel speaker be able to span? I want to build my own but I dont know how much bass it should have. I have a reasonable sub so bass in the system is not a problem. Considering a 2 Hi-Vi F-5 and a singal RT1C A palnar in a vented enslosure. I need to have brilliant highs to match the ESS monitors that use a Heil AMT. I think this setup should be good from about 70 Hz to 22kHz